As a filter device for removing particulate contained in an exhaust gas from a diesel engine, various devices have been proposed conventionally. For instance, U.S. Pat. No. 4,205,971 discloses a device. The device has a ceramic fiber material provided in a cylindrical form in a cylindrical filter case which is disposed close to an exhaust port of internal combustion engine. The ceramic fiber includes a loose ceramic fiber pad as an outer layer and a woven inner ceramic fiber mat. A cavity entending in the axial direction is connected to an exhaust conduit. The exhaust port of the engine is connected to an inlet connecting pipe of the filter case. The ceramic fiber material is supported at its radial interior and exterior by metal perforated plates as needed. A woven metal wire may be provided between the inner ceramic fiber mat and the inner perforated plate.
As the outside fibers in the ceramic fiber pad of this filter are entangled loosely and has poor bonding, the fiber arrangement is disturbed when the exhaust gas directly hits the fibers, thus causing an early reduction of the particulate-collecting function.
Further, since this filter uses super-heated soot particles present in the exhaust gas as an ignition source for burning soot caught by the ceramic fiber material, the filter apparatus should be disposed close to the exhaust port. Generally, however, it is difficult to obtain a large space for the filter apparatus in the vicinity of the engine. While the burning of carbon deposit requires that the temperature be kept at 600.degree. C. or higher for a predetermined time period, the temperature of the exhaust gas varies greatly based on the running conditions of an automobile. Accordingly, it is very difficult to keep such condition. This rises the undesirable back pressure due to incomplete combustion.
Japanese Unexamined Patent Publication No. Hei 2-256812 discloses a filter apparatus which has a plurality of layers of ceramic fiber filter materials and electric heating elements (heaters) alternately wound around an air-permeable support member (metal cylinder having many holes formed therein). In this apparatus, gaps of various sizes extending radially with respect to the air-permeable support member are formed by the ceramic fiber filter material, and when the exhaust gas from the diesel engine passes therethrough, the particulates in the exhaust gas are caught close to the heaters. When the heaters are energized, the captured particulate will be burned efficiently.
At the time of heat generation caused by energizing the heaters to burn the deposits, the vicinity of the burning portion is heated to 1000.degree. C. or higher. The heaters and the ceramic fibers have different coefficients of thermal expansion. Accordingly, the difference between the amount of expansion of the heaters and that of the ceramic fibers becomes too large to be negligible, for instance, when canthal lines (cermet materials which are used as heating members for generating a temperature of up to 1800.degree. C. and are produced by Canthal Gaderius Co., Ltd.) has its coefficient of thermal expansion which is substantially twice as much as that of ceramic. Therefore, for those lines of a length of 300 mm, for example, a deviation of 2 mm would occur in the range between 1000.degree. C. and the normal temperature. Since many layers of the heaters and ceramic fibers are alternately laminated close to one another in the above conventional apparatus, sliding friction occurs therebetween at the time of heat generation caused by energizing the heaters. Accordingly, the ceramic fibers susceptible to friction are liable to be broken or damaged, so that its durability appears poor.
As the heaters are sealed in the ceramic fibers in the conventional apparatus, the temperatures of the heat generating portion and its environmental portion which is in direct contact with the heat generating portion excessively rise, so that the temperature gradient becomes large due to the additional burning heat of deposits accumulated around there. Consequently, not only the ceramic fibers but also the heaters may be damaged by the friction originating from the difference in the coefficient of thermal expansion and the locally high temperature. In the case where either the heaters or ceramic fibers are damaged for some reason and need replacing, it would take time for disassembling or the like to replace only either one while the replacement of the entire unit would result in replacement of the undamaged one, requiring an extra cost.
It is a primary object of the present invention to provide a heat-resistant filter, which will suppress a rise in back pressure to a small amount even when exhaust gas processing is executed for a long time period, will not raise a problem in removing deposits through heating and burning by an electrical heat-generating member, and can endure a corrosive action under a high-temperature acidic atmosphere.
It is a second object to provide a heat-resistant filter, which, in addition to the above object, is very easy to handle, ensures easy reproduction of the filter, ensures automatic and continuous, stable burning and reproduction of deposits without an operation by a driver during driving of an automobile, and, when either one of an electrical heat-generating member or a filter body constituting the filter needs replacing, allows for simple replacement of only the one that should be replaced.